In many electronic applications, it is desirable to reliably reverse the polarity of a high voltage power supply using a low voltage control signal. However, current systems for accomplishing this objective generally involve the use of resonant circuit components, such as transformers, or other elements that introduce unacceptable delays between receipt of the control input and the effect on the controlled output, preventing fast polarity switching. Additionally, these techniques require considerable complexity to control the individual switch elements. Other presently available systems use multiple high voltage sources that are switched in or out via the use of electromechanical relays. While this method is functional, relays are known to impair system reliability.
In the context of an external defibrillator, U.S. Pat. No. 6,175,765 to Sullivan et al. (xe2x80x9cSullivan et al. referencexe2x80x9d) discloses a four-leg H-bridge output circuit that includes a solid-state switch in each leg. By selectively switching on pairs of switches in the H-bridge output circuit, a biphasic defibrillation pulse may be applied to the patient. However, the Sullivan et al. reference teaches that the switches are preferably three silicon-controlled rectifiers (SCRs) and a switch comprised of two insulated gate bipolar transistors (IGBTs). Both types of switches are driven by xe2x80x9can oscillating control signal, preferably a pulse trainxe2x80x9d that must be continuously applied in order to maintain the switch in the xe2x80x9cONxe2x80x9d position. The drive circuits for both types of switches involve resonating elements (e.g., transformers) that will introduce switching delays. Furthermore, the control signal for the IGBT pair must be significantly amplified by its drive circuit in order to change the IGBT pair to the xe2x80x9cONxe2x80x9d state. The Sullivan et al. reference acknowledges that xe2x80x9cthe maximum working voltage of presently available IGBTs is not sufficient to withstand the maximum voltage that may occur across switch SW2.xe2x80x9d It attempts to solve this problem by connecting two IGBTs in series so that the voltage is split therebetween. Accordingly, the Sullivan et al. reference describes a polarity-reversing circuit that requires complex switch-driving circuit with independent switch-driving sub-circuits for each IGBT. The disclosed circuit is, therefore, an expensive way to adapt polarity reversing circuits for use in high-voltage applications. Moreover, the use of SCRs in the bridge network is not practical for low power applications. SCRs require a minimum amount of current (holding current) to flow through them in order for the junction to remain open. In low power applications, the load current could easily fall below the holding current required by the SCR and the switch would no longer be operational. Moreover, as acknowledged in the Sullivan et al. reference, systems used to reverse the polarity of low voltage power sources generally cannot be adapted for use with high voltage power supplies. Adapting polarity-switching circuits intended for low-voltage applications for use in high-voltage applications often involves the substitution of larger, heavier, costlier and/or less efficient electrical components.
U.S. Pat. No. 6,046,551 to Kita (xe2x80x9cKita referencexe2x80x9d) is directed to a device for controlling the supply of high voltage (400 V) power to an igniter circuit for a discharge lamp. The control device includes an H bridge having two pairs of transistors for applying a high AC voltage to the igniter circuit when a DC input is received. The Kita reference requires that the first pair of transistors must have a higher withstand voltage than the second pair of transistors. The Kita reference describes a CPU as creating a bridge control signal for turning on one or the other pair of transistors. Based on FIG. 1 of the Kita reference, the control signal must be sufficient to forward-bias the junction between the drain terminal (which may be coupled to a high voltage line) and the gate terminal. Accordingly, the circuit disclosed in the Kita reference requires that high-voltage control signals be applied to effect polarity reversal. Furthermore, these control signals are not isolated from the remainder of the H bridge.
Accordingly, there is a need for a polarity-reversing circuit adapted for use with a single high voltage power source that can transition from one polarity to the other without introducing significant time delays between application of a control signal and the changing of switch states to effect the reversal. Moreover, there is a need for a polarity-reversing circuit for use with a high-voltage power source that can be controlled by applying a low voltage signal. There is also a need for a polarity reversing circuit that can be configured to reduce ripple (noise) on the output caused by a high voltage switching power supply.